Tropical Storm Karen Forecast Discussion Number 14

By | September 25, 2019

Issued at 1100 AM AST Wed Sep 25 2019


000
WTNT42 KNHC 251450
TCDAT2

Tropical Storm Karen Discussion Number  14
NWS National Hurricane Center Miami FL       AL122019
1100 AM AST Wed Sep 25 2019

Karen is still having a tough time establishing and maintaining a
well-organized structure.  The deep convection which developed over
the center earlier this morning has since collapsed, leaving a
ragged and disorganized cloud pattern.  In addition, a sizable
outflow boundary moving away from the eastern part of the
circulation suggests that there is dry air within the vortex.  The
initial intensity is being held at 40 kt, pending possible
scatterometer data later this morning and a reconnaissance flight
this afternoon.

The initial position is a little difficult to locate, but the best
estimate of the current motion is 360/13 kt.  Karen is moving
northward between a mid-level high centered over the central
Atlantic and a mid- to upper-level low located just east of the
Bahamas.  This low is expected to slowly retrograde westward, with
ridging developing over the western Atlantic by day 3.  This change
in steering will cause Karen to turn northeastward during the next
48 hours but then make a clockwise loop once it is blocked by the
ridge.  After day 3, the ridge should force Karen to move
west-southwestward.  Most of the track models agree on this general
scenario.  There is some latitudinal spread on days 4 and 5 after
the loop occurs, with the HWRF being the most notable outlier by
not showing much of a westward motion.  The consensus aids,
however, have remained fairly steady, and therefore there was no
compelling reason to make any significant changes compared to the
previous track forecast.

Except for the possibility of dry air in the circulation, it's not
quite evident why Karen has not been able to sustain organization.
Still, the cyclone is expected to move beneath an upper-level
anticyclone during the next 24-48 hours, and if the shear does
indeed decrease, then some strengthening would be expected.  There
continues to be a dichotomy among the intensity models, with the
dynamical models (including the GFS and ECMWF) keeping the cyclone
weak while the statistical-dynamical models still show
intensification through days 4 and 5.  It's difficult to ignore
what's being shown by the global models, since there must be
something in the environment that they're deeming to be negative
for continued strengthening.  The best course of action at this
point is to maintain a steady intensity after 48 hours, but it
should be noted that what is shown in the official forecast still
lies above the ICON intensity consensus and the HCCA corrected
consensus aid.

Key Messages:

1.  Karen will continue to produce heavy rainfall, potentially
causing additional flash floods and mudslides, across Puerto Rico
and the Virgin Islands today even as the center moves away from the
region.


FORECAST POSITIONS AND MAX WINDS

INIT  25/1500Z 21.7N  64.9W   40 KT  45 MPH
 12H  26/0000Z 23.4N  64.4W   45 KT  50 MPH
 24H  26/1200Z 25.4N  63.7W   45 KT  50 MPH
 36H  27/0000Z 26.9N  62.9W   50 KT  60 MPH
 48H  27/1200Z 27.5N  62.0W   55 KT  65 MPH
 72H  28/1200Z 26.8N  62.0W   55 KT  65 MPH
 96H  29/1200Z 26.0N  65.0W   55 KT  65 MPH
120H  30/1200Z 25.5N  68.5W   55 KT  65 MPH

$$
Forecaster Berg

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